As a kind of field-effect transistor, a thin film transistor in which a channel formation region is formed in a semiconductor layer formed over a substrate having an insulating surface is known. Techniques in which amorphous silicon, microcrystalline silicon, or polycrystalline silicon is used for the semiconductor layer used in the thin film transistor have been disclosed (Patent Documents 1 to 5). A typical application of a thin film transistor is a liquid crystal television device, and the thin film transistor has been put to the practical use as a switching transistor for each pixel included in a display screen.